The present invention relates to a process for preparing optionally substituted benzyl carbazates by reacting C.sub.1 -C.sub.8 -dialkyl carbonates with optionally substituted benzyl alcohols in the presence of a catalyst, followed by reaction with hydrazine hydrate.
Increasingly, carbazates are being used as intermediates for preparing crop protection agents and pharmaceutics. In particular, they are employed for synthesizing peptides (European Patent Application EP 106,282). Furthermore, according to J. Biol. Chem., 266, 5525 (1991), it is possible to prepare hydrazinosuccinate, an inhibitor of aspartate aminotransferase, from benzyl carbazate. European Patent Application EP 143,078 describes the use of benzyl carbazate for preparing crop protection agents. Against this background, the preparation of carbazates assumes increased importance.
In general, carbazates are obtained by reacting chloroformic esters with hydrazine. Thus, Chem. Ber., 92, 1478 (1959) describes the reaction of benzyl chloroformate with hydrazine with formation of benzyl carbazate.
However, the preparation of the precursors, which is carried out using phosgene, is comparatively complex, and the purity of the resulting carbazates is insufficient for some purposes since by-products that contaminate the desired carbazate are formed during the multistep synthesis from thermally sensitive intermediates.
According to another method, symmetrical dialkyl carbonates are reacted with hydrazine. J. Am. Chem. Soc., 70, 1181 (1948). Here, likewise, the preparation of the precursors is difficult or impossible, particularly for higher carbonates. Moreover, this procedure is problematic since, in the isolation of the product as a hydrochloride, the formation of chlorinated, and in some cases corrosive, compounds during work-up cannot be avoided completely.
Furthermore, European Patent Application EP 770,598 discloses a process for preparing optionally substituted C.sub.2 -C.sub.20 -alkyl, C.sub.3 -C.sub.6 -cycloalkyl, and C.sub.6 -C.sub.12 -aryl carbazates in which an unsubstituted C.sub.1 -C.sub.4 -alkyl carbazate is reacted with an optionally substituted C.sub.2 -C.sub.20 -alkyl, C.sub.3 -C.sub.6 -cycloalkyl, or C.sub.6 -C.sub.12 -aryl alcohol in the presence of a catalyst. Here, the unsubstituted C.sub.1 -C.sub.4 -alkyl carbazate can be obtained by reacting dialkyl carbonates with hydrazine. See, for example, Ber. Chem. Ges., 47, 2183-2188. Whereas the purity of the resulting alkyl, cycloalkyl or aryl carbazates is good (for example, 97%), the carbazate yield of the process, based on the alkyl carbazate employed, is relatively low (for example, only about 65%).
Due to the growing demand for carbazates, particularly for benzyl carbazates, it was therefore an object to provide a technically simple process for preparing benzyl carbazates of high purity.